1. Field of the Invention
The present invention relates to two-port non-reciprocal circuit devices. In particular, the present invention relates to a two-port non-reciprocal circuit device, such as an isolator, used in a microwave band and to a communication apparatus.
2. Description of the Related Art
A two-port isolator is disclosed in Japanese Unexamined Patent Application Publication No. 2004-88744 (Patent Document 1) as a two-port non-reciprocal circuit device in the related art. A basic equivalent circuit of the two-port isolator is shown in FIG. 15. In a two-port isolator 301, one end of a first central electrode L1 is electrically connected to an input terminal 314 via an input port P1. The other end of the first central electrode L1 is electrically connected to an output terminal 315 via an output port P2.
One end of a second central electrode L2 is electrically connected to the output terminal 315 via the output port P2. The other end of the second central electrode L2 is grounded via a ground port P3. A parallel RC circuit including a matching capacitor C1 and a resistor R is electrically connected between the input port P1 and the output port P2. A matching capacitor C2 is electrically connected between the output port P2 and the ground port P3.
The first central electrode L1 and the matching capacitor C1 define a first LC parallel resonant circuit and the second central electrode L2 and the matching capacitor C2 define a second LC parallel resonant circuit. In the above-described circuit configuration, since the first LC parallel resonant circuit between the input port P1 and the output port P2 does not resonate and only the second LC parallel resonant circuit resonates when a signal is transmitted from the input port P1 to the output port P2, the insertion loss is reduced.
The insertion loss and isolation are typically important among electrical characteristics required of the non-reciprocal circuit device. Requirements for the insertion loss and the isolation depend on a communication system, the configuration of a communication circuit, and/or functions added to a mobile phone. A comparison between the requirements and actual characteristics can produce a situation in which the requirements are sufficiently met in terms of the insertion loss but are not met in terms of the isolation, or a situation in which the requirements are sufficiently met in terms of the isolation but are not met in terms of the insertion loss.
If the inductance of the second central electrode L2 is increased in the two-port isolator 301 in the related art, the forward transmission characteristics in a broader band, having a reduced insertion loss, are yielded although the bandwidth of the isolation characteristics is narrowed.
However, if the inductance of the central electrode L2 is set so as to exceed a predetermined value by any of the following three methods, problems are caused and it becomes impossible to flexibly adjust the insertion loss characteristics.
(1) If the central electrode L2 is lengthened, the ferrite is enlarged in accordance with the increasing length of the central electrode L2. As a result, the product cannot be reduced in size.
(2) If the line width of the central electrode L2 is narrowed, the equivalent series resistance of the central electrode L2 is increased and the Q factor of the central electrode (inductor) L2 is decreased. As a result, the insertion loss is increased.
(3) When the central electrode L2 is wound around the ferrite, the interval of the central electrodes is shortened as the number of wind ings thereof is increased and, thus, short circuit frequently occurs. If the windings of the central electrode are sufficiently spaced such that the short circuit does not occur, the product cannot be reduced in size.
In addition, if the inductance of the central electrode L2 is set so as to exceed the predetermined value, the capacitance of the capacitor C2 with which the central electrode L2 defines the parallel resonant circuit is substantially reduced in a relatively high-frequency system, such as Personal Communication Services (PCS) (having a center frequency of 1,880 MHz) or Wideband Code Division Multiple Access (W-CDMA) (having a center frequency of 1,950 MHz). Accordingly, it is difficult to measure and adjust the capacitance and, therefore, it is not possible to mass-produce the product. In addition, there are situations in which the stray capacitance is greater than the required capacitance, and it is not possible to actuate the isolator 301 at a desired frequency. Furthermore, there are also situations in which the electrical length of the central electrode L2 is greater than λ/4 and the central electrode L2 does not function as an inductor. In this situation, the parallel resonant circuit cannot be provided.